It is already known, for example, to prepare polyisocyanates having biuret groups from diisocyanates and water as described in German Pat. No. 1,101,394 and U.S. Pat. No. 3,201,372; hydrogen sulphide as described in German Pat. No. 1,165,580 and British Pat. No. 1,043,672; formic acid as described in German Pat. No. 1,174,764 and U.S. Pat. No. 3,392,183 or tertiary alcohols as described in German Pat. No. 1,543,178 and U.S. Pat. No. 3,358,010. In these reactions, amino groups are first formed from part of the isocyanate groups of the diisocyanates put into the process, and these amino groups react with excess diisocyanate to form the polyisocyanates having biuret groups by way of the corresponding urea diisocyanates. These prior art processes have numerous disadvantages. In a heterogeneous reaction of diisocyanates with water, there is a risk of the formation of insoluble polyureas which are difficult to separate out. Furthermore, these prior art processes are always accompanied by the formation of gaseous by-products such as carbon dioxide, carbon monoxide, carbon sulphoxide or olefines. One particularly serious disadvantage of these prior art processes is that part of the isocyanate groups of the diisocyanate used as starting material is invariably destroyed by amine formation. There have, therefore, been no lack of attempts to prepare polyisocyanates having biuret groups by direct reaction of polyamines with polyisocyanates without release of volatile by-products or destruction of isocyanate groups with amine formation. However, all these attempts have met with considerable practical difficulties because of the high reactivity of the amino groups with isocyanate groups, and the formation of insoluble polyureas and cross-linked products is, therefore, very high. The only processes which met with some success were, therefore, those in which very specialized starting materials were used. In German Auslegeschrift No. 1,215,365 and U.S. Pat. No. 3,441,588, for example, it is necessary to use higher molecular weight diaminopolyethers as the diamine component in order to eliminate formation of the above-mentioned difficultly soluble by-products. It goes without saying that the necessity of using diaminopolyethers which must first be prepared by a complicated process cannot provide a commercially completely satisfactory solution to the problem. The process according to German Offenlegungsschrift No. 1,963,190 and U.S. Pat. No. 3,824,266 is restricted to the use of diprimary aromatic diamines which are reduced in their reactivity by steric or electronic effects.
The process according to German Offenlegungsschrift No. 2,261,065 and U.S. Pat. No. 3,903,126 also provides no commercially practicable way of producing polyisocyanates having biuret groups by direct reaction of organic polyisocyanates with simple aliphatic and/or cycloaliphatic polyamines.
Thus, in Example 16 of German Offenlegungsschrift No. 2,261,065 and U.S. Pat. No. 3,903,126 in which polyisocyanates having biuret groups are prepared from hexamethylene diisocyanate and hexamethylene diamine, it is necessary to reheat the reaction mixture to 180.degree. C. for 12 hours to complete the reaction. This long period of reheating at a high temperature is not only uneconomical but causes discoloration of the reaction product, particularly under large scale commercial production conditions, so that the possiblity of using this product in lightfast lacquers is significantly limited.
Although polyisocyanates having a biuret structure, and particularly those based on hexamethylene diisocyanate, have achieved a position of world-wide commercial importance for the production of lightfast and extremely weather-resistant lacquers with maximum gloss retention, these polyisocyanates are manufactured and marketed with as low a proportion of monomeric diisocyanate as possible. Extensive toxicological investigations and many years of experience in the processing of these products have shown that the upper permissible limit of monomer content in these polyisocyanates is about 0.7%, (hexamethylene diisocyanate), based on the solids content, because only then is it possible to ensure that lacquers obtained from these products can be applied without risk to health, and even then only if the usual protective measures for lacquer processing are observed. This upper limit of about 0.7% has been accepted in the literature, e.g. in the leaflet "PUR-Anstrichstoffe" of the Hauptverbandes der deutschen gewerblichen Berufsgenossenschaft and in "Polyurethane Report" of the Paintmakers Association.
Recent extensive investigations have shown that if the aforementioned polymolecular polyisocyanate mixtures having a biuret structure are stored for long periods, particularly under uncontrolled conditions, for example, when transported by ship to hot countries, etc., this limit of about 0.7% of monomeric hexamethylene diisocyanate is exceeded and can easily rise to over about 1% due to catalytic reactions with the walls of the containers if the products are packaged in glass or metal containers. These changes are due to unknown catalytic effects and impurities which cannot be exactly identified analytically, and they also depend on the temperature, which may vary, e.g. between about 20.degree. and 50.degree. C.
Since it is in practice impossible to keep to the monomer limited to about 0.7% in the production of the aforementioned polyisocyanates and the conditions for safe processing of such products has been etablished for more than 10 years, it is of the greatest importance for both commercial and ecological reasons to increase the stability of these products and reduce their rate of decomposition back into monomers and at the same time to reduce the viscosity of the known biuret polyisocyanates based on aliphatic or cycloaliphatic diisocyanates, which is often in the region of about 10,000 to 120,000 cP at 20.degree. C. Such a reduction in the viscosity makes it possible for one and two-component polyurethane lacquers to be produced without solvent. Although a process for the production of exceptionally low viscosity polyisocyanates having a biuret structure, for example biuret polyisocyanates based on hexamethylene diisocyanate, has already been described in U.S. Pat. No. 3,903,127, the polyisocyanates obtainable by this process also have the disadvantage of releasing monomeric hexamethylene diisocyanate during prolonged storage.
It was, therefore, an object of the present invention to provide a process by which polyisocyanates having biuret groups could be prepared substantially without the disadvantages of the processes known in the prior art.
It was surprisingly found that this object could be achieved by using mixtures of water and primary amines as "biuretizing agents". By "biuretizing agents" are meant substances which react with organic isocyanates to form biuret groups.